I have a number of patents on respiratory calorimeters. Broadly, these devices calculate the oxygen consumption of a user by subtracting the exhaled flow volume, less the exhaled CO.sub.2, from the inhaled flow volume. Some of these operate by integrating the flow volume of a number of inhalations and exhalations over a period of time and by subtracting the CO.sub.2 volume in the exhalation from the integral of the exhaled volume by scrubbing the CO.sub.2 and then subtracting the exhaled flow volume less the CO.sub.2 volume from the inhaled flow volume to determine oxygen consumption during the period. I also have a pending application that measures both inspired and expired volume and either O.sub.2 or CO.sub.2 content to determine oxygen consumption. The carbon dioxide scrubber is bulky and requires replenishment after a number of uses. Carbon dioxide or oxygen analyzers are also relatively expensive.
It has previously been proposed to determine the mass of a gas flowing through a conduit by determining the transit time of ultrasonic pulses passed through the gas in a direction having a component along the axis of flow so as to determine the flow rate of the gas, and additionally determining the density of the gas. U.S. Pat. No. 2,911,825 discloses such a system in which the acoustic impedance of the gas is measured to determine the density. U.S. Pat. No. 5,214,966 similarly employs the transit time of ultrasonic pulses to determine the flow rate and determines the density of the flowing gas through measurement of the velocity of sound through the gas. U.S. Pat. No. 5,645,071 uses the transit time of ultrasonic pulses to determine the flow rate and additionally makes temperature measurements which, with the flow rate, allow the determination of mass of the flowing gas. This latter patent also suggests the application of this device to pulmonary function diagnostics and discloses an additional gas analyzing sensor for determining the carbon dioxide and/or oxygen content of the flowing gas on an on-line, real time basis.
It would be desirable to provide a method of analysis which allows the determination of oxygen consumption, carbon dioxide production and related and derived respiratory factors without the need for any gas analyzers, such as O.sub.2 and CO.sub.2 analyzers. This would result in a low cost, high precision instrument suitable for a wide range of health care applications.